Magnetic recording mediums comprising a non-magnetic support having thereon a magnetic layer containing ferromagnetic iron oxide powder or ferromagnetic metal or alloy powder (hereinafter referred to simply as "ferromagnetic metal") dispersed in a binder are conventionally used as magnetic recording mediums for video, audios and computers.
The high density and long-time recording ability of the magnetic recording mediums have been improved year after year. Higher image quality and higher sound quality are highly demanded, and it is necessary to improve the electromagnetic characteristics. To meet these demands, the output of the magnetic recording mediums must be increased and the noise must be reduced. Further improvement is necessary in hardware, such as the VTR. It is considered that higher output can be obtained by using ferromagnetic metal having a high coercive force and high saturation magnetization (.sigma.s), increasing a loading density with more finer ferromagnetic powder, or improving the surface properties to reduce spacing loss formed by the magnetic head. Further, it is considered that a thinner magnetic layer or support is needed to cope with long-time recording.
As a method for improving image quality and sound quality, there have been proposed multilayer magnetic recording mediums comprising an upper layer containing ferromagnetic metal and a lower layer containing ferromagnetic iron oxide powder, whereby functions are separated to improve electromagnetic characteristics over a wide range [see, JP-A-54-145104 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-58-56231, JP-A-60-256917 and JP-A-2-110823]. In the method described in JP-A-2-110823, fine ferromagnetic iron oxide particles are used in the lower layer to improve surface properties, thus ultimately improving the electromagnetic characteristics. However, the charging characteristics are poor, and hence the running properties deteriorate.
As a method for obtaining excellent running properties, the magnetic layer generally contains carbon black having an average primary particle diameter of 10 to 150 m.mu.. In general, when carbon black having a small particle size is used, the deterioration of the surface properties of the resulting magnetic layer is relatively small. Further, use of small carbon black particles result in poor running properties. When carbon black having a large particle size is used, running properties are good, but smoothness is inferior. Hence, the electromagnetic characteristics are lowered. For this reason, attempts have been made to use a combination of carbon black particles having various average primary particle sizes in the magnetic layer when the magnetic layer is composed of a single layer [see, JP-B-54-9041 (the terms "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-53-20203 and JP-A-58-218039]. However, a magnetic recording medium having good running properties, as well as good electromagnetic characteristics, has not be obtained.
In the case of multi-layer magnetic recording mediums comprising two or more magnetic layers, there have been disclosed methods wherein different types of carbon blacks are added to the upper layer and the lower layer as a means for obtaining magnetic recording mediums having excellent running properties, as well as good electromagnetic characteristics (see, JP-A-58-200425, JP-A-63-300425 and JP-A-1-205726). In the specification of JP-A-58-200425, carbon black is contained only in the second magnetic layer (the upper layer) to improve the electromagnetic characteristics. In the specifications of JP-A-63-300425 and JP-A-1-205726, coarse carbon black particles are contained in the second magnetic layer (the upper layer) and fine carbon black particles are contained in the first magnetic layer (the lower layer) to improve the antistatic properties and running properties without lowering the electromagnetic characteristics.
In the system wherein the upper layer contains ferromagnetic metal and the lower layer contains cobalt-modified iron oxide, the charging characteristics are greatly deteriorated. Additionally, the content of the fine carbon black particles to be contained in the lower layer must be high and the electromagnetic characteristics are lowered. Further, when the magnetic layer is made thinner by taking long-term recording into consideration, the surface properties are deteriorated and the electromagnetic characteristics are lowered.
In general, when the coercive force is increased, output in the region of short wavelengths is increased, but output in the region of long wavelengths is lowered. There is proposed the use of a magnetic layer composed of a multi-layer wherein the upper layer contains ferromagnetic metal and the lower layer contains magnetic iron oxide powder. In this system, it is contemplated that output can be improved over a wide range by making the recording of signals in the region of long wavelengths on the lower layer and using ferromagnetic powder having a high coercive force in the upper layer.
In order to obtain good electromagnetic characteristics, it is necessary that the surface properties of the magnetic layer are improved to thereby reduce a spacing loss between the magnetic head and the tape. Particularly, when ferromagnetic metal is used, saturated magnetization is high. As a result, it is difficult for the powder to be dispersed due to the magnetic interaction between magnetic particles. In the absence of high saturated magnetization, or, the particles are apt to agglomerate even when the particles are dispersed. Accordingly, good surface properties can not be obtained. Thus, many dispersion methods, dispersants and binders have been proposed. Further, there have been proposed methods wherein after the magnetic layer is coated and dried, the surface thereof is smoothed by passing it between a metallic roll and a resilient roll or metallic rolls.
It is desirable that the magnetic recording mediums have excellent electromagnetic characteristics as well as excellent running durability. In particular, the smoother the surface of the magnetic layer, the more difficulty there is in imparting excellent running durability. To solve the problem, the magnetic layer is generally composed of carbon black. Alternatively, an abrasive such as corundum, silicon carbide or chromium oxide is added thereto. However, considerable amounts of these powders must be used to obtain the effect of imparting good running durability. By adding considerable amounts of such powders, the loading degree of the ferromagnetic powder is lowered and the surface properties of the magnetic recording mediums are deteriorated. Accordingly, good electromagnetic characteristics can not be obtained.
Further, there have been proposed methods wherein fatty acids, or esters of fatty acids with aliphatic alcohols, are contained as lubricants in the magnetic layer to thereby reduce the coefficient of friction. However, a larger amount of the lubricant must be used to provide a low coefficient of friction on the smooth magnetic layer. The molecular weight of the lubricant is very small in comparison with the binder. Hence, when a large amount of the lubricant is used, the magnetic layer is plasticized, and as a result, still durability is deteriorated.
JP-A-2-206020 discloses a multi-layer structure wherein magnetic iron oxide powder is used in the upper layer as well as in the lower layer. The stearic acid adsorption amount of ferromagnetic powder is specified, and ferromagnetic powder having a lower stearic acid adsorption amount is used in the upper layer. The purpose of the above properties is so that even when the amount of the fatty acid to be added to the magnetic layer is small, the fatty acid exudes on the surface of the tape and functions as a lubricant without being adsorbed by ferromagnetic powder. However, a potential problem has been found in that in the system wherein the upper layer contains metal and the lower layer contains cobalt-modified iron oxide, the magnetic layer has a high coefficient of friction, wherein the coefficient is in the range of the stearic acid adsorption amount described in the specification of JP-A-2-206020. The coefficient of friction is further increased after repeated running.